It's going to depend on its save, but given that these models rarely have less than a 4+, and buffs often aren't too difficult to come by, I fear it'l be a pain to knock down a peg or two. And I dislike the general armsrace it results in of bringing bigger and badder stuff to try and get the first blow in. Also, slaanesh being stupidly fast getting in the first blow is going to be a pain.
Also, What really bothers me is that there's no real counter beyond "strike first". I can't send in say a horde of fodder and have it waste it's fancy 5 damage attacks cuz it'l just wipe out that horde in a few swipes. Nor can I send a capable duellist to lock him down as he'l just get crushed unless he gets the first few blows in. The more new stuff is released for AoS the more I want a mechanic to distinguish multi-target and single-target attacks (like 40k's wounds not carrying over rule) so we can field counters beyond "attack first" or "just shoot it".
A similar issue holds for the new things like dark temptations. There's no choice to make as the target, the chance of dying is so high that the potential benefits do not even begin to compare to the price, but refusing the bargain deals significant damage, especially since you'l also be in range for the keeper to finish you off immeadiatly. So as the target it's not a matter of choosing to avoid temptation or give in, it's just a matter of pick your poison. With the only genuine "counter" being just never being in range for it (or being tzeentch and having cheat dice I guess). The idea is really neat, but the negative effects are so significant thatthere just isn't any temptation...